Brillouin laser

ABSTRACT

Techniques for producing a Brillouin laser are provided. According to some aspects, techniques are based on forward Brillouin scattering and a multimode acousto-optic waveguide in which light is scattered between optical modes of the waveguide via the Brillouin scattering. This process may transfer energy from a waveguide mode of pump light to a waveguide mode of Stokes light. This process may be referred to herein as Stimulated Inter-Modal Brillouin Scattering (SIMS). Since SIMS is based on forward Brillouin scattering, laser (Stokes) light may be output in a different direction than back toward the input pump light, and as such there is no need for a circulator or other non-reciprocal device to protect the pump light as in conventional devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Patent Application No. 62/504,868, filed May 11, 2017,titled “Brillouin Laser in Silicon,” which is hereby incorporated byreference in its entirety.

GOVERNMENT FUNDING

This invention was made with government support under grant numberDGE1122492 awarded by the National Science Foundation Graduate ResearchFellowship and under grant number N00014-16-1-2687 awarded by Office ofNaval Research. The government has certain rights in the invention.

BACKGROUND

The ability to shape and control light using silicon has enabled adiverse array of chip-scale applications within the burgeoning field ofsilicon photonics. The potential for emerging technologies that benefitfrom highly customized light sources has spurred great interest insilicon-based nonlinear laser oscillators as a way to reshape thespectral and coherence properties of on-chip light. For example, Ramanand Kerr nonlinearities have been harnessed to create all-silicon Ramanlasers and for demonstrations of long-wavelength Kerr frequency combs insilicon resonators.

One intriguing class of laser oscillators is based on Brillouininteractions, which are produced by the coupling between light andsound. Brillouin interactions are often exceptionally strong, overtakingKerr and Raman nonlinearities in most transparent media. However, thesilicon waveguides that radically enhance Raman and Kerr nonlinearitiesand are used to produce Raman lasers or Kerr frequency combs produceexceedingly weak Brillouin couplings.

SUMMARY

According to some aspects, a Brillouin laser is provided comprising aclosed loop acousto-optical waveguide, an optical input arranged toinput pump light into the closed loop acousto-optical waveguide, and anoptical output, distinct from the optical input, arranged to outputlaser light from the closed loop acousto-optical waveguide.

According to some embodiments, the optical input is arranged to inputthe pump light in a first forward direction, and the optical output isarranged to output the laser light in a second forward direction.

According to some embodiments, the first forward direction and thesecond forward direction are parallel directions.

According to some embodiments, the Brillouin laser further comprises adirectional coupler configured to receive the pump light from theoptical input, couple the pump light to the closed loop acousto-opticalwaveguide, and couple laser light from the closed loop acousto-opticalwaveguide to the optical output.

According to some embodiments, the closed loop acousto-optical waveguidesupports at least two optical modes.

According to some embodiments, the at least two optical modes comprise asymmetric optical mode and an antisymmetric optical mode.

According to some embodiments, the closed loop acousto-optical waveguidecomprises a racetrack cavity.

According to some embodiments, the closed loop acousto-optical waveguidecomprises a cavity formed on a substrate, and wherein one or moreportions of the cavity are suspended over void regions of the substrate.

According to some embodiments, the Brillouin laser further comprises aplurality of tethers mechanically supporting the cavity in the one ormore portions.

According to some embodiments, the closed loop acousto-optical waveguidecomprises a semiconductor cavity.

According to some embodiments, the closed loop acousto-optical waveguidehas a circumference between 100 μm and 10 cm.

According to some embodiments, the closed loop acousto-optical waveguidesupports acoustic modes in some, but not all, of a closed loop of theclosed loop acousto-optical waveguide.

According to some aspects, a method of producing light using a Brillouinlaser is provided, the method comprising providing pump light into aclosed loop acousto-optical waveguide, the pump light being input to anoptical input of the closed loop acousto-optical waveguide, andproducing laser light from the closed loop acousto-optical waveguide,the laser light being output from the closed loop acousto-opticalwaveguide through an optical output of the closed loop acousto-opticalwaveguide, distinct from the optical input of the of the closed loopacousto-optical waveguide.

According to some embodiments, the pump light is input in a firstforward direction, and the optical output is output in a second forwarddirection.

According to some embodiments, the first forward direction and thesecond forward direction are parallel.

According to some embodiments, the pump light and the laser light havedifferent frequencies.

According to some embodiments, a difference between frequencies of thepump light and the laser light is equal to the closed loop acousto-opticwaveguide's Brillouin frequency.

According to some embodiments, the method further comprises selecting afrequency of the pump light based at least in part on the closed loopacousto-optic waveguide's Brillouin frequency.

According to some embodiments, the closed loop acousto-optical waveguidesupports at least two optical modes.

According to some embodiments, the closed loop acousto-optical waveguidecomprises a semiconductor cavity.

The foregoing apparatus and method embodiments may be implemented withany suitable combination of aspects, features, and acts described aboveor in further detail below. These and other aspects, embodiments, andfeatures of the present teachings can be more fully understood from thefollowing description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIG. 1 depicts a schematic of an illustrative Brillouin laser, accordingto some embodiments;

FIGS. 2A-2B depict requirements for phase matching and energyconservation in an acousto-optic waveguide, according to someembodiments;

FIG. 3A is a schematic of an illustrative Brillouin laser comprising aracetrack cavity, according to some embodiments;

FIG. 3B depicts transfer of energy between modes of the illustrativewaveguide of FIG. 3A, according to some embodiments;

FIG. 3C depicts transmission spectra of symmetric and anti symmetricmodes of the illustrative waveguide of FIG. 3A, according to someembodiments.

FIGS. 4A-4C depict an illustrative implementation of a Brillouin laser,according to some embodiments;

FIG. 5 depicts an illustrative directional coupler, according to someembodiments;

FIGS. 6A-6C depict an alternative Brillouin laser configuration,according to some embodiments;

FIGS. 7A and 7B depict illustrative resonant amplifiers, according tosome embodiments;

FIGS. 8A-8C illustrate a mode multiplexer, according to someembodiments; and

FIG. 8D depicts an illustrative device that may be operated as aresonantly enhanced amplifier or as a multimode optical parametricoscillator, according to some embodiments.

DETAILED DESCRIPTION

Conventional Brillouin lasers suffer from a number of deficiencies. Forinstance, Brillouin lasers that have been demonstrated require extremelyfine tuning of waveguide dimensions to properly amplify pump light.These devices are also typically based on backward Stimulated BrillouinScattering (SBS), which produces amplified light in a direction opposingthe pump light. This configuration poses a significant challenge fordevice integration, since such devices require a high quality circulatoror other non-reciprocal device to protect the pump light source fromunwanted feedback.

Conventional Brillouin lasers may also suffer from the production ofmultiple Stokes orders when the pump power is increased. Because suchdevices rely on the pump and Stokes light propagating in the samespatial mode, when the Stokes wave becomes strong enough it can itselfreach the lasing threshold and can thereby produce cascaded energytransfer to successive Stokes orders. The conventional approach isthereby also limited in the manner in which a desired output may beefficiently created from the pump light.

The inventors have recognized and appreciated new techniques forproducing a Brillouin laser. These techniques are based on forwardBrillouin scattering and a multimode acousto-optic waveguide in whichlight is scattered between optical modes of the waveguide via theBrillouin scattering. This process leads to energy transfer from awaveguide mode of the pump light to a waveguide mode of the Stokeslight. This process may be referred to herein as Stimulated Inter-ModalBrillouin Scattering (SIMS). Since SIMS is based on forward Brillouinscattering, laser (Stokes) light may be output in a different directionthan back toward the input pump light, and as such there is no need fora circulator or other non-reciprocal device to protect the pump light asin conventional devices.

According to some embodiments, energy transfer between modes of aBrillouin laser based on SIMS may be tunable, allowing precise controlover Stokes light produced within the waveguide. For instance, the powerthreshold required for a Stokes wave to cascade to another Stokes ordercan be altered as desired. In some cases, cascading can be completelysuppressed by engineering the modes of the waveguide such that higherorder Stokes waves are not supported by any mode of the waveguide. Assuch, energy may be transferred from the pump light to Stokes lightwithin the waveguide without inadvertently creating energy transfer tosuccessive Stokes orders as in conventional devices based on SBS.

According to some embodiments, a Brillouin laser based on SIMS may beimplemented on-chip. For instance, a Brillouin laser comprising anon-chip silicon waveguide may be fabricated in some embodiments. Anon-chip approach may facilitate a Brillouin laser that is both small andmonolithic, and that may be easily configured using standard siliconfabrication techniques to engineer the modes of the waveguide byfabricating the waveguide with particular dimensions.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, techniques for producing a Brillouinlaser. It should be appreciated that various aspects described hereinmay be implemented in any of numerous ways. Examples of specificimplementations are provided herein for illustrative purposes only. Inaddition, the various aspects described in the embodiments below may beused alone or in any combination, and are not limited to thecombinations explicitly described herein.

FIG. 1 depicts a schematic of an illustrative Brillouin laser, accordingto some embodiments. Brillouin laser 100 comprises an acousto-opticwaveguide 110 that has an input 111 and an output 112. In operation,pump light 101 may be supplied to the input 111 and laser (Stokes) light102 is output from the output 112. The acousto-optic waveguide 110 has acircumference L.

As used herein, an “acousto-optic waveguide” refers to a waveguide thatsupports at least one optical mode and at least one acoustic (phonon)mode, although according to some embodiments the at least one acousticmode need not be supported around the entire loop of the waveguide. Forinstance, the acousto-optic waveguide may support one or more opticalmodes around its circumference whilst supporting one or more acousticmodes around part, but not all, of the circumference. Since, as will bedescribed below, the acoustic modes of the waveguide mediate energytransfer between optical modes of the waveguide, it is not a requirementthat such energy transfer occurs in all parts of the waveguide, and as aresult, in some cases only portions of the waveguide may support one ormore acoustic modes.

In Brillouin laser 100, optical self-oscillation requires opticalfeedback and sufficient optical gain to compensate for round-tripoptical loss. Optical gain in the laser 100 is supplied by stimulatedBrillouin scattering within the waveguide 110, which is a phase-matchednonlinear process that produces stimulated optical spatial gain for theStokes wave. Thus, optical self-oscillation ensues within the waveguide110 when the Stokes spatial mode reaches a lossless state, a conditionthat occurs when round-trip optical gain balances round-trip opticalloss. To produce the optical gain, energy is transferred between opticalmodes of the waveguide, driven by the pump light 101 and by Brillouinscattering (photon-phonon scattering) processes within the waveguide. Asa result, it is important that the waveguide 110 supports at least oneacoustic mode. Optical gain may be output from the waveguide as laser(Stokes) light 102.

According to some embodiments, acousto-optic waveguide 110 may befabricated from any suitable material or combination of materials solong as the waveguide supports multiple optical modes and at least oneacoustic mode as described above. It will be appreciated that numeroustechniques for fabricating a waveguide structure may be employed,although in particular crystal and/or crystalline materials may beparticularly suitable since both light and acoustic phonons maypropagate through such a structure. Illustrative materials that may besuitable for fabrication of the acousto-optic waveguide 100 may includesilicon, AlGaAs, AlN, Si₃N₄, Ge, SiO₂ (silica glass), doped silica,CaF₂, chalcogenides, or combinations thereof.

According to some embodiments, acousto-optic waveguide 110 may beconfigured to support at least two optical modes, and in particular maybe configured to support at least two transverse optical modes. Asdiscussed above, a Brillouin laser as described herein may transferenergy from a first mode (e.g., a first transverse optical mode) to asecond mode (e.g., a second transverse optical mode) of the waveguidevia Stokes processes. Such energy transfer may occur in either directionbetween transverse optical modes of the waveguide. For instance, awaveguide supporting three transverse optical modes may transfer energyfrom any one of these modes to any other one of these modes (e.g., fromfirst to second, from first to third, from second to first, from secondto third, from third to first, or from third to second).

In some embodiments, acousto-optic waveguide 110 may be configured tosupport at least two optical modes that include a symmetric optical modeand an antisymmetric optical mode. In operation, pump light may initiatea transfer of energy from a first mode (e.g., the antisymmetric opticalmode) to a second mode (e.g., the symmetric optical mode) of thewaveguide via Stokes processes.

Irrespective of the particular form of the optical modes of theacousto-optic waveguide 110, the waveguide and pump light may meetcertain criteria for optical amplification to occur, as described below.

In the case that a first transverse mode and a second transverse mode ofthe acousto-optic waveguide 110 have different propagation constants,k₁(ω) and k₂(ω), respectively, the resonance conditions, k₁(ω)L=2πm andk₂(ω)L=2πn, where L is the circumference of the acousto-optic waveguide110, produce a distinct set of resonant frequencies ω₁ ^(m) and ω₂ ^(n).In the following description, subscripts of 1 and 2 will be used todenote the first transverse mode and the second transverse mode,respectively, and superscripts m and n are used for the modes' resonanceindices, respectively.

Through inter-modal Brillouin scattering, a pump wave ω_(p) traveling inthe second transverse mode of the acousto-optic waveguide 110 mayproduce amplification of the Stokes wave ω_(s) propagating in the firsttransverse mode of the acousto-optic waveguide 110. According to someembodiments, this may occur when the following energy conservation andphase matching conditions are satisfied:

ω_(p)=ω_(s)+Ω_(b)

k ₂(ω_(p))=k ₁(ω_(s))+q(Ω_(b))

Here, q(Ω) is the acoustic dispersion relation and Ω_(b) is theBrillouin frequency of the acousto-optic waveguide 110, which is set bythe physical dimensions and shape of the waveguide. Together theseconditions require:

k ₂(ω_(p))=k ₁(ω_(p)−Ω_(b))+q(Ω_(b))  (Eqn. 1)

These requirements for phase matching and energy conservation arerepresented by FIGS. 2A and 2B. Phonons that satisfy the condition ofEqn. 1 lie on the acoustic dispersion curve, q(Ω) shown in FIG. 2B andconnect the initial (open circle) and final (solid circle) opticalstates identified on the optical dispersion curve of FIG. 2A. From thisdiagram, we see that a backward-propagating phonon mediates Stokesscattering between pump and Stokes photons propagating in the secondtransverse and first transverse modes, respectively. By contrast, theanti-Stokes process is mediated by a forward propagating phonon. Sincedistinct phonon wavevectors mediate Stokes and anti-Stokes processes,these interactions are decoupled, permitting single-sidebandamplification. Laser oscillation of the first transverse cavity mode (ω₁^(m)) occurs when Brillouin gain matches the round-trip loss, producingcoherent laser emission at the Stokes frequency (ω_(s)=ω₁ ^(m)).

According to some embodiments, the above-described lasing requirementsmay be met by inputting pump light of power P_(p) into the acousto-opticwaveguide 110 through input 111 as shown in FIG. 1. As the input pumpwave 101 is tuned to ω₂ ^(n), pump light resonantly circulates in then^(th) second transverse cavity mode. When this pump power exceeds athreshold power (P_(p)>P_(th)), the Stokes field builds from thermalnoise to yield appreciable line-narrowing and coherent Stokes emissionat a frequency ω_(s)=ω_(p)−Ω_(b). This Stokes light may be output fromthe acousto-optic waveguide 110 through output 112. As a result of theabove-described process, the input pump light 101 may produce laserlight output 102.

According to some embodiments, the first transverse optical mode of theacousto-optic waveguide 110 may be a symmetric optical mode, and thesecond transverse optical mode of the acousto-optic waveguide 110 may bean antisymmetric optical mode. In the discussion that follows, the firsttransverse optical mode of the acousto-optic waveguide is presumed to bea symmetric optical mode and the second transverse optical mode of theacousto-optic waveguide is presumed to be an antisymmetric optical mode.This discussion should not be viewed as limiting, however, as thetechniques described herein are not limited to energy transfer betweenthese particular types of transverse optical modes. As discussed above,a Brillouin laser as described herein may transfer energy from any firstoptical mode to any second optical mode of the waveguide via Stokesprocesses. As a result, it will be appreciated that the particularillustrative transverse optical modes described below are providedmerely as one example.

According to some embodiments, the acousto-optic waveguide 110 mayinclude one or more components to couple the pump light to the waveguideand/or to couple the Stokes light from the waveguide to output 112.Generally, such components may allow the Stokes light to couple to theoutput 112 without substantially coupling the pump light within thewaveguide to the output. In some embodiments, the acousto-opticwaveguide may include a directional coupler configured to couplestrongly to one mode of the waveguide (e.g., the second transverse modeof the waveguide) and to couple weakly to another mode of the waveguide(e.g., the first transverse mode of the waveguide).

According to some embodiments, the acousto-optic waveguide 110 maycomprise any one or more semiconductor materials and/or any othermaterials capable of supporting acoustic phonon modes. While thewaveguide is not limited to any particular material(s) or arrangementsof said material(s), it will be appreciated that there may be advantagesto fabricating the waveguide from commonly-used semiconductor materialsused in on-chip fabrication such as, but not limited to, silicon.

FIG. 3A is a schematic of an illustrative Brillouin laser comprising aracetrack cavity, according to some embodiments. In the example of FIG.3A, Brillouin laser 300 comprises racetrack cavity 310, to which inputpump light is provided via input 311 and coupler 315. The coupler 315also couples Stokes light produced in the cavity to an output 312.Racetrack cavity 310 is an example of acousto-optic waveguide 110 thatis arranged as a racetrack shape and that includes a coupler forreceiving pump light and outputting laser (Stokes) light, as discussedabove. In the example of FIG. 3A, the racetrack cavity 310 is a ridgewaveguide.

In the example of FIG. 3A, the coupler 315 is configured to be adirectional coupler that couples strongly to the antisymmetric mode ofthe waveguide 310 and weakly to the symmetric mode of the waveguide 310.

In the example of FIG. 3A, symmetric and antisymmetric modes of lightare illustrated at various points along the input, bus, output andwaveguide by single peak and double-peak waveforms, respectively. Forinstance, the input pump light 311 is coupled via coupler 315 to thedepicted antisymmetric mode of the waveguide 310. Within the waveguide,energy is transferred from the antisymmetric mode into the symmetricmode, as mediated by acoustic phonons in the waveguide. The resultingStokes light is output from the waveguide via coupler 315 into the bus316. The Stokes light entering the bus from the symmetric mode of thewaveguide is represented in FIG. 3A by the smaller, secondary waveformsand by the smaller arrow in output 312. It will be appreciated that theinput and bus in the example of FIG. 3A may be waveguides having anynumber of modes and that the depiction of multiple waveforms in thefigure is provided merely to illustrate the propagation of the lightthrough the system. In some embodiments, the input 311 and/or bus 316comprise single-mode waveguides.

In the example of FIG. 3A, the waveguide 310 includes two regions inwhich the waveguide is suspended. These regions, one of which is labeledas suspended region 321, are shaded dark grey and are located along thestraight portions of the racetrack waveguide. In some embodiments,suspended sections of a waveguide may be beneficial to tightly confineboth light and sound, and to thereby enable a sufficiently strongBrillouin coupling that mediates the transfer of energy between modes ofthe waveguide.

The transfer of energy from the antisymmetric mode to the symmetric modeof the illustrative waveguide 310 in region 330 is depicted in FIG. 3B.In FIG. 3B, the physical extent of the region 330 is shown along thehorizontal axis (labeled “position(z)”), and the relative optical powerof the antisymmetric and symmetric modes is shown in the vertical axis.The upper portion of FIG. 3B conceptually depicts the acoustic phononmediated transfer of energy from the antisymmetric mode (shown with acomparatively high amplitude double peak waveform on the left of thefigure) to the symmetric mode (shown with a comparatively high amplitudesingle peak waveform on the right of the figure).

FIG. 3C depicts transmission spectra of symmetric and antisymmetricmodes of the illustrative waveguide of FIG. 3A, according to someembodiments. In FIG. 3C, elements 361-363 relate to the symmetric mode,371-373 relate to the antisymmetric mode, and 381-383 relate to thecombined modes of the waveguide, as discussed below.

In the example of FIG. 3C, plot 362 depicts the transmission spectrum ofthe symmetric mode with cavity resonances denoted by the letter m,whereas plot 372 depicts the transmission spectrum of the antisymmetricmode with cavity resonances dented by the letter n. As shown in FIG. 3C,energy transfer from the antisymmetric mode to the symmetric mode may beachieved when ω_(p) (the pump light frequency) is selected such thatω_(p) corresponds to a resonance of the symmetric mode and there is acorresponding frequency ω_(s)=ω_(p)−Ω_(b) that is a resonance of theantisymmetric mode. This relationship between ω_(p) and ω_(s) isdepicted in FIG. 3C by the two vertical dotted lines.

As may be seen in FIG. 3C, lasing behavior is produced for particularfrequencies of the pump light ω_(p). In some cases, the free spectralranges (FSRs) of the two optical modes of the cavity may be differentsuch that there are numerous such frequencies ω_(p) that produce lasingbehavior.

While acousto-optic waveguide 110 may be fabricated in any of numerousways, FIGS. 4A-4C depict one illustrative implementation of theBrillouin laser 100, according to some embodiments. In the example ofFIGS. 4A-4C, an acousto-optic waveguide 400 is formed as a racetrackcavity 410 from silicon on a silica substrate 422. Along straightregions of the racetrack, the silicon is suspended above void regions(of which void region 425 is one example). According to someembodiments, the Brillouin laser 400 may be fabricated from asingle-crystal silicon-on-insulator (SOI) wafer.

In the example of FIGS. 4A-4C, a plurality of tethers (of which tether427 is one example) mechanically support the racetrack cavity 410 oneither side of the suspended regions. The array of tethers areidentified in the figure by the solid black lines. According to someembodiments, presence of the void regions beneath the racetrack cavity410 may have a negligible impact on the guidance of optical modes of thecavity, yet may enable guidance of a phonon mode that mediates efficientBrillouin coupling between the optical modes of the cavity.

As discussed above, lasing behavior may be produced only for particularfrequencies of the pump light ω_(p) that meet the energy conservationand phase-matching conditions of Eqn. 1. In the illustrative device ofFIGS. 4A-4C, the racetrack has a circumference of 4.576 cm and exhibitscavity free spectral ranges (FSRs) of 1.614 GHz and 1.570 GHz for thesymmetric and antisymmetric optical modes of the cavity, respectively.For this device, the resonance frequency conditions are satisfied bysymmetric and antisymmetric cavity mode pairs that occur every 0.40 nmacross the C-band (from 1530-1565 nm), corresponding to an approximatefrequency difference between pairs of around 50 GHz. According to someembodiments, the Brillouin gain in the Brillouin laser 400 of FIGS.4A-4C may be substantially larger than for conventional Brillouinlasers. For instance, the gain may be on the order of 400 W⁻¹ m⁻¹.

FIG. 5 depicts an illustrative directional coupler, according to someembodiments, that may be configured to couple light as an input to, andan output from, an acousto-optic waveguide, such as waveguide 100 shownin FIG. 1, waveguide 300 shown in FIG. 3A, and/or waveguide 400 shown inFIG. 4A. In the example of FIG. 5, pump light 511 is supplied at a portB, and light in both symmetric and antisymmetric modes propagatesthrough the acousto-optic waveguide 510 (of which only a portion isdepicted for clarity).

In the example of FIG. 5, directional coupler 515 (which may also bereferred to as a mode multiplexer coupler) is configured to transfermost, or all, of the pump light into the antisymmetric mode of theacousto-optic waveguide 510, whilst light propagating in the symmetricmode of the acousto-optic waveguide 510 remains in that mode. In someembodiments, the directional coupler 515 may transfer light into and/orout of the acousto-optic waveguide 510 via a phase-matched coupling.

In some embodiments, the directional coupler 515 may couple to thesymmetric mode of the waveguide, at least to some extent. In some cases,this coupling may be due to crosstalk between the coupler's coupling tothe symmetric and antisymmetric modes of the waveguide. Irrespective ofhow the directional coupler 515 is coupled to the symmetric mode of thewaveguide, as a result light propagating in the symmetric mode of theacousto-optic waveguide 510 may be transferred into the coupler tooutput 512. Thus, Stokes (laser) light propagating in the waveguide maybe output from the waveguide to output 512.

According to some embodiments, directional coupler 515 may comprise, ormay be comprised of, silicon. In some implementations, the directionalcoupler 515 may be a ridge waveguide (e.g., of the same cross-sectionalshape as the ridge waveguide shown in the example of FIGS. 4A-4C).According to some embodiments, a height of the directional coupler 515is the same as, or approximately the same as, the height of theacousto-optic waveguide 510 to which it is coupled. For instance, thedirectional coupler 515 may comprise a ridge waveguide of height 215 nm(relative to a 135 nm silicon layer for non-waveguide portions of thechip), and the acousto-optic waveguide 510 may also comprise of a ridgewaveguide of height 215 nm (relative to the 135 nm silicon layer).Non-limiting and illustrative dimensions for the directional coupler 515shown in FIG. 5 may be as follows: w₁=1.5 μm; L_(c)=32 μm; w₂=630 nm.

FIGS. 6A-6C depict an alternative Brillouin laser configuration,according to some embodiments. FIGS. 6A and 6B illustrate the behaviorof a mode multiplexer element, labeled “MM” in the figures, and FIG. 6Cdepicts a Brillouin laser 651.

In illustrative Brillouin laser 651, the pump light does not circulatewithin waveguide 610. Rather, the input pump light 611 is coupled into atransverse optical mode (hereinafter referred to as transverse mode 2)of the waveguide via a mode multiplexer (labeled “MM” in the figures).

FIGS. 6A and 6B illustrate the basic operation of the mode multiplexer(MM). Light coupled into port 1 of the mode multiplexer is multiplexedinto transverse mode 1 of the multimode optical waveguide (FIG. 6A). Thelight can then be demultiplexed out of port 1 using an identical modemultiplexer. FIG. 6B illustrates the same principles for transverseoptical mode 2. Light coupled into port 2 of the mode multiplexer ismultiplexed into transverse optical mode 2 and then demultiplexed by anidentical mode multiplexer.

In illustrative Brillouin laser 651, the pump light 611 input to modemultiplexer 615 amplifies the Stokes wave as it traverses aBrillouin-active region of the waveguide (that is, a region thatsupports at least one acoustic phonon mode). The pump light is thenoutput 612 from the waveguide via mode multiplexer 616. In this device,the waveguide is effectively transparent to the pump light, because thepump light passes directly through a portion of the waveguide loop. Assuch, it is not required in the example of FIGS. 6A-6C that the pumplight satisfy the cavity resonance condition.

It will be appreciated that, although the above-described techniqueshave been described in the context of a Brillouin laser, the techniquesmay also be directed in other contexts. As one example, FIG. 7A depictsan illustrative resonant amplifier, according to some embodiments.

In the example of FIG. 7A, device 701 operates to amplify an inputsignal wave at the Stokes frequency. In the configuration shown in FIG.7A, a signal wave 713 (Stokes “seed”) of frequency ω_(s) is injected viacoupler 722 and resonantly amplified through stimulated inter-modalBrillouin scattering, resulting in an amplified signal wave coupled outvia coupler 721 and output 714. As with Brillouin laser 651 shown inFIG. 6C, device 701 is non-resonant for the pump wave (transparent tothe pump), meaning that pump wave does not circulate around the loop ofthe waveguide. Rather, the pump light 711, after being multiplexed intotransverse mode 2 of the acousto-optic waveguide via mode multiplexer715, and amplifying the signal wave (at Stokes frequency), isdemultiplexed and output 712 from the device through port 2 of theintegrated mode multiplexer 716. In this device, the input and outputcoupler regions for the signal (Stokes) wave 721 and 722 are distinctfrom the mode multiplexers. These couplers couple signal light into andout of the symmetric mode of the optical waveguide.

Illustrative device 701 may also operate as an inter-modal Brillouinlaser (see FIG. 6C) if there is no signal wave input and if theround-trip Brillouin gain is sufficient to balance the round-tripoptical loss. If the device operates above threshold and there is aninput signal wave present, the system may operate as an injection-lockedBrillouin laser.

FIG. 7B depicts a second illustrative resonant amplifier, according tosome embodiments. In device 751, a multi-mode racetrack ring resonatoroperates to amplify input light at the Stokes frequency. An injectedsignal wave (Stokes “seed”) of frequency ω_(s) 762 is resonantlyamplified through stimulated inter-modal Brillouin scattering, resultingin an amplified signal wave at the output 763. In this configuration,the input signal 762 and pump wave 761 are coupled into the resonatorusing a mode multiplexer (MM) and a symmetric coupler that couples thesignal and pump waves into transverse modes 1 and 2 of the multimodewaveguide, respectively. In contrast to the configuration of device 701shown in FIG. 7A, the pump wave is resonantly enhanced within theracetrack ring cavity. The amplified signal wave is coupled out of theracetrack ring resonator through a mode- or wavelength-selectivedirectional coupler 770. For example, a multimode two point coupler maycouple out the signal wave (in transverse mode A) with minimal crosstalkfrom the pump wave.

According to some embodiments, a mode multiplexer may be configured tomultiplex and/or demultiplex any number of optical transverse modes thatare supported by a multimode optical waveguide. For instance, asillustrated in FIGS. 8A-8C, a multiplexer may have 3 inputs, eachcorresponding to a distinct transverse mode. As a result, light providedto a particular input of the multiplexer couples to a respective opticalmode of an optical waveguide, and the respective optical mode of thewaveguide may also couple to a particular output of the multiplexer.

FIG. 8D depicts a device 809 that may be operated as a resonantlyenhanced amplifier or as a multimode optical parametric oscillator,according to some embodiments. In the example of FIG. 8D, device 809 maybe operated such that a single pump wave simultaneously amplifies twoinput signal waves 821 and 831 (of frequencies ω_(s) and ω_(s2)respectively), provided there are supported phonon modes within theacousto-optic waveguide that mediate each amplification process (e.g.,at least two phonon modes). According to some embodiments, the twosignal waves 821 and 831 may resonate within distinct optical cavitiesas shown that both share an active waveguide region 810. In the exampleof FIG. 8D, device 809 is non-resonant (transparent) to the pump wave(input 811, output 812), meaning that the pump wave does not circulatewithin the resonators.

According to some embodiments, device 809 may also be configured toproduce parametric amplification in a four-wave mixing process. In thiscase, input 821 may be a signal wave and input 831 may be an idler waveof frequency ω_(i). This process, which relies upon Kerr nonlinearities,requires that both phase matching and energy conservation be conserved.In the illustrative configuration of FIG. 8D:

2ω_(p)=ω_(s)+ω_(i)

2k ₂(ω_(p))=k ₁(ω_(s))+k ₃(ω_(i))

where k₁, k₂ and k₃ are propagation constants for the three modes of thewaveguide.

This system may operate as an amplifier for either the signal or idlerwaves if there is an input signal or idler wave, respectively. Thesystem may also operate as an optical parametric oscillator (laser) ifno inputs for the signal or idler waves are provided.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Further, some actions are described as taken by a “user.” It should beappreciated that a “user” need not be a single individual, and that insome embodiments, actions attributable to a “user” may be performed by ateam of individuals and/or an individual in combination withcomputer-assisted tools or other mechanisms.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A Brillouin laser, comprising: a closed loopacousto-optical waveguide; an optical input arranged to input pump lightinto the closed loop acousto-optical waveguide; and an optical output,distinct from the optical input, arranged to output laser light from theclosed loop acousto-optical waveguide.
 2. The Brillouin laser of claim1, wherein the optical input is arranged to input the pump light in afirst forward direction, and wherein the optical output is arranged tooutput the laser light in a second forward direction.
 3. The Brillouinlaser of claim 2, wherein the first forward direction and the secondforward direction are parallel directions.
 4. The Brillouin laser ofclaim 1, further comprising a directional coupler configured to receivethe pump light from the optical input, couple the pump light to theclosed loop acousto-optical waveguide, and couple laser light from theclosed loop acousto-optical waveguide to the optical output.
 5. TheBrillouin laser of claim 1, wherein the closed loop acousto-opticalwaveguide supports at least two optical modes.
 6. The Brillouin laser ofclaim 5, wherein the at least two optical modes comprise a symmetricoptical mode and an antisymmetric optical mode.
 7. The Brillouin laserof claim 1, wherein the closed loop acousto-optical waveguide comprisesa racetrack cavity.
 8. The Brillouin laser of claim 1, wherein theclosed loop acousto-optical waveguide comprises a cavity formed on asubstrate, and wherein one or more portions of the cavity are suspendedover void regions of the substrate.
 9. The Brillouin laser of claim 8,further comprising a plurality of tethers mechanically supporting thecavity in the one or more portions.
 10. The Brillouin laser of claim 1,wherein the closed loop acousto-optical waveguide comprises asemiconductor cavity.
 11. The Brillouin laser of claim 1, wherein theclosed loop acousto-optical waveguide has a circumference between 100 μmand 10 cm.
 12. The Brillouin laser of claim 1, wherein the closed loopacousto-optical waveguide supports acoustic modes in some, but not all,of a closed loop of the closed loop acousto-optical waveguide.
 13. Amethod of producing light using a Brillouin laser, the methodcomprising: providing pump light into a closed loop acousto-opticalwaveguide, the pump light being input to an optical input of the closedloop acousto-optical waveguide; and producing laser light from theclosed loop acousto-optical waveguide, the laser light being output fromthe closed loop acousto-optical waveguide through an optical output ofthe closed loop acousto-optical waveguide, distinct from the opticalinput of the of the closed loop acousto-optical waveguide.
 14. Themethod of claim 13, wherein the pump light is input in a first forwarddirection, and wherein the optical output is output in a second forwarddirection.
 15. The method of claim 14, wherein the first forwarddirection and the second forward direction are parallel.
 16. The methodof claim 13, wherein the pump light and the laser light have differentfrequencies.
 17. The method of claim 16, wherein a difference betweenfrequencies of the pump light and the laser light is equal to the closedloop acousto-optic waveguide's Brillouin frequency.
 18. The method ofclaim 13, further comprising selecting a frequency of the pump lightbased at least in part on the closed loop acousto-optic waveguide'sBrillouin frequency.
 19. The method of claim 13, wherein the closed loopacousto-optical waveguide supports at least two optical modes.
 20. Themethod of claim 13, wherein the closed loop acousto-optical waveguidecomprises a semiconductor cavity.